Drilling operations often employ a number of techniques to gather information such as the depth and inclination of a borehole and the types of rocks through which a drill pipe and drill bit are drilling. For this purpose, techniques called Logging-While-Drilling (LWD) and Measurement-While-Drilling (MWD) were developed in the oil exploration and production industry, which enable the collection of data in real-time. LWD collects logging information similar to the conventional wireline logging, while MWD also enables a driller to determine the position and orientation of the drill bit and direction of a borehole during the drilling operation so that the driller can more accurately control the drilling operations.
MWD permits data to be transmitted in complex wells, such as directional and horizontal drills. MWD is achieved by incorporating MWD tools into a module in the steering tool of the drill string. These MWD tools collect real time drilling information, and transmit this data as pulses through the mud column. This data is transmitted to the surface and can be used for real-time data acquisition, geosteering, and formation evaluation.
Common to MWD techniques is the problem of transmitting data from the bottom of a borehole to a point on the surface where it can be collected and processed. One technique for this type of data transmission is mud pulse telemetry. During the drilling operation, drilling mud is pumped from a mud pump downward through the drill pipe and emerges near the drill bit at the bottom of the drill hole. This mud cools and lubricates the drill bit, carries rock cuttings to the surface where they can be analyzed and prevents the walls of the borehole from collapsing.
In mud pulse telemetry, a transmission device, or “pulser,” such as an electro-mechanical pulser or a mud siren near the drill bit generates an acoustic signal that is transmitted upward to the surface through the downward traveling column of mud. Modern mud sirens, for example, are capable of generating a carrier pressure wave of 12 Hz. A transducer, generally at the surface, receives the signal and transmits it to a signal processor. The signal processor then decodes and analyzes the signal to provide information about the drilling operation to the driller.